Backward compatible head and head positioning assembly for a linear digital tape drive

ABSTRACT

A tape recording and playback unit records and reads back data from tape either in a high track density standard format or alternatively reads back data from a tape previously recorded on another unit in accordance with a previous low density standard track format. The unit includes a primary head positioning mechanism for positioning a primary multi-channel write/read head and a secondary head positioning mechanism for positioning a secondary read-only head. The secondary head may be positioned angularly to read longitudinal and azimuth track patterns, and also to be retracted when not needed.

REFERENCE TO RELATED APPLICATION

This is a continuation application of U.S. patent application Ser. No.09/149,733, filed on Sep. 8, 1998, now U.S. Pat. No. 6,188,532, andentitled: “BACKWARD COMPATIBLE HEAD AND HEAD POSITIONING ASSEMBLY FOR ALINEAR DIGITAL TAPE DRIVE”. The contents of U.S. patent application Ser.No. 09/149,733 are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to tape drives, and more particularly thepresent invention relates to a linear digital tape drive having abackward-compatible auxiliary head and head positioning assemblyenabling read back of outdated standard tape formats.

BACKGROUND OF THE INVENTION

Magnetic tape is widely used for recording digital information. Oneextensive use of digital tape recording is to provide backup andarchival storage of vast quantities of digital information, such asrecords comprising blocks of data. In some applications archival recordsare recorded on tape in a particular tape format which follows agreedstandards at the time the recording was made. The tape may then beplaced into archival storage and not retrieved until months or yearshave passed by. It is not uncommon to specify the useful storage life ofrecorded digital tapes and cartridges at thirty years, or longer.Whatever may be the useful life of a particular magnetic tapes, aprimary assumption on the part of those who store such tapes away isthat the recorded information may be read at some date in the future, ifaccess to the archived data is required.

While a particular tape and cartridge may remain functional over manyyears after being in archival storage, tape transport mechanismstypically do not last nearly so long. Standardized tape recordingformats are also susceptible to evolutionary changes and improvements.These changes are primarily driven by improvements in magnetic tape andmagnetic head technologies which enable much larger data records andfiles to be stored on a given area of magnetic tape. One recentdevelopment, first employed in the hard disk drive industry, and morerecently applied to tape recording, has been the introduction of headassemblies formed of thin film inductive, and magneto-resistive, andgiant magneto-resistive (MR) read elements. These elements are typicallyfabricated in processes including photolithographic patterning steps ofthe type first developed for use by the semiconductor industry. Onedesirable aspect of these new thin film MR heads is that head gap widthsmay be narrowed considerably. Narrower head gaps and finer grainmagnetic media coatings on tape mean that many more lineal data tracksmay be defined across a magnetic recording tape of a standard givenwidth (such as one-half inch tape). Also, the head structure may beformed as a single small composite structure on a common base orsubstrate and have as many as 12, or more, distinct heads. By using acommon substrate, the heads may be formed to be in a predeterminedprecise alignment relative to nominal track locations defined along themagnetic tape. With e.g. 12 write and read head elements of the headstructure in precise alignment with the defined nominal track locations,and with large scale integrated chips providing multiple data write/readchannels, it has now become practical to have e.g. 12 channels forsimultaneously writing user data to tape and for reading user data backfrom tape. This increase in the number of write/read channelseffectively increases the overall data transfer rate between a hostcomputer and the tape drive, and enables the tape drive to becharacterized as having higher performance than previously available.

In order to take full advantage of the new thin film MR head technologyin tape drives, a track layout which differs from previous standardtrack formats is required. This new track layout employs tracks of muchnarrower track width and pitch. Since the write/read heads are groupedtogether on a common fabrication substrate, the data tracks are alsogrouped together. In one arrangement, the data tracks are grouped intobands, or zones, across the tape, such that e.g. ten lateral headpositions relative to the tape within a single zone would access 120tracks. When a zone boundary is reached, the head structure or assemblyis then displaced laterally relative to the tape travel path to the nextzone, and the tracks of that zone then become accessible. Because trackwidths are very narrow, enabling track densities of e.g. 2000 tracks perinch, or higher, lateral tape motion must be followed in order to keepthe new head assemblies in alignment with the tracks during tape travelpast the head. Magnetic servo patterns written onto the tape may be readby servo readers and used to generate position error signals used by aclosed loop positioner to correct head position. Alternatively, opticalservo patterns embossed or otherwise formed on a back side of the tapemay be used to provide position error signals, as disclosed for examplein commonly assigned, co-pending U.S. patent application Ser. No.09/046,723 filed on Mar. 24, 1998, and entitled: “Multi-Channel MagneticTape System Having Optical Tracking Servo”, the disclosure thereof beingincorporated herein by reference.

The later high-density track format differs from previous standardformats. For example, FIG. 1 shows an existing standard tape formatemploying longitudinal recording. In this example a magnetic recordingtape 10 has a series of parallel longitudinal tracks. Three tracks 12A,12B and 12C are shown in the FIG. 1 example, although more tracks, suchas 24, 48, 96 or 128 tracks may be employed in a one-half inch tapelineal format in accordance with a particular standardized track layoutplan. A head assembly 14 includes e.g. discrete inductive read or writehead elements 14A, 14B and 14C which are aligned with the tracks 12A,12B and 12C. Other tracks may be accessed by displacing the headassembly 14 laterally relative to the direction of the tape along a pathindicated by the vertical arrows axial aligned with the head 14 in theFIG. 1 view.

Another preexisting standard tape format employs azimuth recording ofthe data tracks, i.e. adjacent tracks are recorded with magnetic gapsoblique to each other, creating what appears generally as a“herringbone” pattern, shown in FIG. 2. Therein, one track 16A has itsmagnetic flux reversal pattern aligned with a first azimuth angleoblique to the tape travel direction, and an adjacent track 16B has itsmagnetic flux reversal pattern aligned with a second azimuth angle in anopposite sense of the first angle relative to a travel path of themagnetic tape 10. One known advantage derived from azimuth recording isthat lineal guard bands or regions between tracks may be reduced, andthe tracks may be placed closer together and read back withoutinterference from adjacent tracks. While azimuth recording technologyincreases track density somewhat, complications arise in writing andreading the slanted tracks. Multi-element tape heads, such as the tapehead 100 shown in FIGS. 4-6 of U.S. Pat. No. 5,452,152, can be providedwith some of the write/read elements having magnetic gaps aligned withone azimuth angle, and other write/read elements having magnetic gapsaligned with the other azimuth angle. Such heads are then positionedlaterally relative to the direction of tape travel in order to come intoalignment with particular tracks. An alternative approach, also shown inFIG. 2 and enabling compatibility with both the longitudinal tracks 12A,12B and 12C of the FIG. 1 example, and the azimuth tracks 16A and 16B ofthe FIG. 2 example, calls for rotating a head 19 having perpendicularhead elements 19A and 19B between the two azimuth formats and thelongitudinal format. One example of a multi-element head is given incommonly assigned, co-pending U.S. patent application Ser. No.08/760,794, now U.S. Pat. No. 6,867,339, filed on Dec. 4, 1996, andentitled: “Four Channel Azimuth and Two Channel Non-AzimuthRead-After-Write Longitudinal Magnetic Head”, the disclosure thereofbeing incorporated herein by reference. An example of an azimuth taperecording pattern and an apparatus for writing the pattern in accordancewith servo information read back from an adjacent track is given incommonly assigned U.S. Pat. No. 5,371,638, the disclosure thereof beingincorporated by reference.

FIG. 3 illustrates a newer track format plan employing a tape 10Acarrying high recording density magnetic media. According to the FIG. 3track plan, a multiplicity of data tracks 20 n are distributed acrosse.g. five zones 22A, 22B, 22C, 22D and 22E. A monolithic thin film headelement 24 within the head assembly includes e.g. 12 write-read elementsin relatively close proximity enabling writing to and reading fromtracks of a particular zone, e.g. zone 22D in the FIG. 3 example. Otherzones may be accessed by displacing the head assembly laterally relativeto the direction of travel of tape 10A. Further details of a tape andtape drive in accordance with this general approach may be found in theabove-referenced U.S. patent application Ser. No. 09/046,723.

While the standardized longitudinal recording patterns shown in the FIG.1 example, and the azimuth recording patterns shown in the FIG. 2example, have worked very well for a number of years, newer higherdensity track layout patterns and plans, enabled by multi-element thinfilm head as well as improvements in tape media technologies are nowproposed and will most likely become standard approaches in the futurefor certain categories of longitudinal digital tape recording methodsand devices. Since extensive cartridge handling equipment in use iscapable of handling standard cartridges containing tape having the newerformat, no compelling need has arisen to change the cartridge formfactor or major features in order to accommodate the new tape trackformats enabled by emerging new technologies. Yet, a hitherto unsolvedneed has remained for backward compatibility within tape drive unitshaving monolithic multi-element heads by enabling reading back of olderpreexisting tape formats recorded on tape carried in standard tapecartridges, but based on discrete head elements, in order to recoverarchival data recorded on the older tapes.

SUMMARY OF THE INVENTION WITH OBJECTS

A general object of the present invention is to provide a backwardcompatible head and head positioning assembly within a linear digitaltape drive in a manner overcoming limitations and drawbacks of priorapproaches.

Another object of the present invention is to enable a linear digitaltape drive primarily adapted to recording and reading back of trackpatterns of standard cartridge tape recorded in a higher density trackformat to also be able to read back older lower density track patternsof archival standard cartridge tape in order to be able to retrievearchived user data.

Yet another object of the present invention is to provide a secondaryhead positioning and read-only tape head module for backwardcompatibility in reading tape recorded in a low density format andcarried in standard tape cartridges as well as to provide a primary headpositioning and write-read tape head module for forward compatibility inreading tape recorded in a high density format and carried in the sametype of standard tape cartridges.

One more object of the present invention is to provide a “button-shaped”multi-element magnetic recording head which is capable of contacting amagnetic tape at a very slight tape wrap angle, and which may be rotatedbetween positions aligning a magnetic recording gap of an element of thehead with both longitudinal and azimuthal recording patterns of a linealdata track recorded on the tape.

One more object of the present invention is to provide a tape headhaving side wings and dimensions less than tape width such that the tapehead floats in close proximity to a tape with minimized contact,ensuring effective operation with both longitudinal and azimuthalrecording patterns as well as minimal wear and reliable long usefullife.

Accordingly, a tape recording and playback unit is provided forrecording and playing back digital data recorded along a multiplicity ofparallel longitudinal data tracks of a magnetic storage tape. The tracksare arranged in accordance with a standardized high density track layoutin which the tracks have much smaller track widths and are much moreclosely spaced together than tracks defined by older lower densitystandard tape track formats. The unit includes a base, and has a take-upreel. In one preferred form, the unit receives a single reel cartridgeand couples to an outer end of a tape supply held on a supply reel ofthe cartridge and threads the tape along a tape path defined by pluralguide rollers within the unit until the take-up reel is reached.

In order to write to and read from tape tracks in accordance with thestandardized high density track layout, the unit is equipped with aprimary head positioning mechanism. The primary mechanism is referencedto the base and presents a multi-channel primary write/read headassembly to the tape along the tape path. A coarse servo, preferablyincluding a lead screw and nut follower provides coarse elevationalcontrol to the primary write/read head assembly. A fine position servo,preferably including a voice coil motor carried on a body of the nutfollower provides fine adjustments to head position in accordance withposition error signals. Most preferably, the position error signals areprovided via an optical sensor reading optical servo patterns formed ona back side of the high density tape.

In order to provide backward compatibility with lower density standardtape track layouts, a secondary head positioning mechanism is alsoprovided within the unit. The secondary head positioning mechanismsupports and positions a read-only secondary head assembly relative tothe tape. The secondary head positioning mechanism also preferablyincludes a coarse positioner for elevational positioning. In onepreferred form, the secondary head positioning mechanism also includes amechanism for rotating the head to enable read back of longitudinalrecording, azimuth recording, and to assume a retract position when atape recorded with a lower density standard track pattern is notpresent. Most preferably, the coarse positioner of the secondarymechanism is mechanically coupled to the coarse positioner of theprimary mechanism, in order to eliminate a second coarse positionermotor. Coupling via spur gearing between the two mechanisms is presentlypreferred. The electronics of the unit may be switched between theprimary mechanism and the secondary mechanism, based upon sensing aparticular track format standard type. In one preferred form, formatsensing is by way of a unique structural feature provided on anotherwise standard tape cartridge, such that the feature distinguishesbetween high density and low density tape track standard formats.

A secondary read-only head body has a dimension less than a width of thetape and employs a minimized tape wrap angle. The head body has sidewings enabling the head to “float” adjacent to the tape at the minimumwrap angle and effectively operate at longitudinal as well as azimuthplay back angles.

These and other objects, advantages, aspects, and features of thepresent invention will be more fully appreciated and understood uponconsideration of the following detailed description of preferredembodiments presented in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the Drawings:

FIG. 1 is a highly diagrammatic elevational view of a segment ofmagnetic data storage tape recorded with a series of lineal tracksemploying longitudinal recording in accordance with a first preexistingindustry standard tape format.

FIG. 2 is a highly diagrammatic elevational view of a segment ofmagnetic data storage tape recorded with a series of lineal tracksemploying azimuth recording in accordance with a second preexistingindustry standard tape format.

FIG. 3 is a highly diagrammatic elevational view of a segment ofmagnetic data storage tape recorded with a multiplicity of high densitylineal tracks in accordance with a new high-density recording format.

FIG. 4 is simplified diagrammatic plan view of a tape drive and datatape cartridge wherein the tape drive includes a backward-compatiblehead and head positioning mechanism adapted to read the first and secondpreexisting industry standard tape formats as well as a high-densitymulti-channel head and head positioning mechanism adapted to read andwrite the new high-density recording format, in accordance withprinciples of the present invention.

FIG. 5 is an enlarged isometric view of the backward-compatible head andhead positioning mechanism shown in the FIG. 4 tape drive, in accordancewith principles of the present invention.

FIG. 6 is a top plan view of the FIG. 5 head and positioning mechanism.

FIG. 7 is a back side view in elevation of the FIG. 5 head andpositioning mechanism.

FIG. 8 is a side view in elevation of the FIG. 5 head and positioningmechanism.

FIG. 9A is a front view in elevation of the FIG. 5 head and positioningmechanism in a perpendicular orientation relative to tape travel forplayback of longitudinally recorded data tracks in accordance with theFIG. 1 format.

FIG. 9B is a front view in elevation of the FIG. 5 head and positioningmechanism in a first azimuth orientation relative to tape travel in e.ga forward direction for playback of one set of azimuthally recorded datatracks in accordance with the FIG. 2 format.

FIG. 9C is a front view in elevation of the FIG. 5 head and positioningmechanism in a second azimuth orientation relative to tape travel in areverse direction for playback of a second set of azimuthally recordeddata tracks in accordance with the FIG. 2 format.

FIG. 9D is a front view in elevation of the FIG. 5 head and positioningmechanism in a third azimuthal orientation assumed by the head at a headretract position.

FIG. 10 is an enlarged diagrammatic plan view of the retract mechanismof the FIG. 5 head and positioning mechanism.

FIG. 11 is a diagrammatic view in elevation of the spur-gear couplingarrangement between the lead screws of the main head positioningmechanism and the backward-compatible positioning mechanism of the FIG.4 tape drive.

FIG. 12 is a simplified electrical block diagram of the FIG. 4 tapedrive.

FIG. 13 is an enlarged isometric view of the backward-compatible head ofthe FIG. 4 tape drive.

FIG. 14 is an enlarged front view in elevation of the FIG. 13 head.

FIG. 15 is an enlarged side view in elevation of the FIG. 13 head.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to the drawings, where like reference numerals designate likeor corresponding parts throughout the views, FIG. 4 presents a tape unit100 and single-reel tape cartridge 102. The cartridge 102 includes asupply reel 104 and a pancake 106 of spooled magnetic recording tape10A, capable of being written at a high track density. Tape cartridge102, while occupying the same physical envelope or form factor as priorstandards cartridges, has at least one unique structural feature, suchas a uniquely located “beginning of tape” (BOT) hole in accordance e.g.with commonly assigned U.S. Pat. No. 5,790,337 to Steinberg et al., andentitled: “Tape Cartridge Qualified by Location, and Identified byGeometry, of Type Aperture”, the disclosure thereof being incorporatedherein by reference. Alternatively, the cartridge 102 may be providedwith a structural feature comprising a uniquely located notch 107, foridentifying the tape 10A as high density recording tape. A tape-typesensor 109 associated with a cartridge receiver portion of the tape unit100 may be provided to sense the notch 107 and thereby to informelectronics of the unit of the particular tape type. Other physicalfeatures, such as an embossed optical servo pattern formed on a backside of the tape may also provide a unique structural feature forindicating a high track density tape. When a unique BOT hole, or thenotch 107, is not sensed at a particular cartridge, the unit 100 isalerted that the tape format (if any) of the particular cartridge is inaccordance with a prior standards track format, and calls for use of anauxiliary read-only capability present within the unit 100. An outer endof the tape pancake 106 is buckled by a suitable buckling mechanism to aleader extending from a take-up reel 108 of the tape unit 100. Apresently preferred form of tape buckling mechanism is describe incommonly assigned, co-pending U.S. Pat. No. 5,971,310, filed on the samedate as this application and entitled: “Positive Engagement Buckle for aTape Drive and Cartridge”, the disclosure thereof being incorporatedherein by reference. An alternative form of buckling mechanism isdescribed in commonly assigned U.S. Pat. No. 5,769,346 to Daly, andentitled: “Tape Buckling Mechanism for Single Reel Cartridge TapeRecording”, the disclosure thereof being incorporated herein byreference. A tape supply reel motor 242 and a take-up reel motor 244 areprovided in the tape unit 100 (see FIG. 12) but are not shown in theFIG. 4 diagrammatic plan view.

Four tape guide rollers 110, 112, 114 and 116 guide the tape 10A fromthe supply reel 106 to the take-up reel 108. Two of the rollers 110 and112 are formed on a frame 118, and two of the rollers are mounted to aframe 120. The frames 118 and 120 are secured to a base 122 of the unit110. A primary head positioning mechanism 124 is secured to the base 122at a location between guide rollers 112 and 114. The primary positioningmechanism 124 includes a primary multi-channel write/read head assembly126 for writing and reading user data onto and from the tape 10A inaccordance with a standardized higher density track layout, e.g. of theFIG. 3 type. The mechanism 124 also includes a frame 128 supporting arotating primary lead screw 130. A stepper motor 132, shown in FIG. 11,rotates the primary lead screw 130. A primary head block 134 displacesthe primary head 126 laterally across the tape 10A as the primary leadscrew 130 is rotated by the stepper motor 132.

In this particular tape unit 100, the high density tape 10A includeslongitudinal servo patterns or tracks formed on the back side thereofe.g. during manufacturing. An optical servo head 134 also mounted to theprimary head block 134 adjacently faces a back side of the tape 10A andoptically senses the longitudinal servo patterns in order to generateposition error signals which are fed into a fine position servo loop(FIG. 12) and result in voice coil driving currents applied to operate aprimary voice coil motor 136, also a part of the primary head block 134.During data writing and reading operations, the fine position servo loopkeeps the primary head 126 in alignment with the data track locations inthe presence of disturbances, such as lateral tape motions andvibrations imparted to the tape 10A along the tape path. As alreadynoted, the optical servo patterns formed on the back side of a highdensity tape 10A may be sensed to determine that a particular cartridgecontains high density format tape.

Also present in the FIG. 4 plan view is a secondary head positioningmechanism 140. As may be seen in greater structural detail in FIGS. 5-9,the secondary mechanism 140 includes a frame 142 which may be secured tothe base 122 of the tape unit 100. Preferably, the mechanism 140 isformed as a unitary module which may be attached to and removed from thebase 122 as a single unit. Suitable electrical plugs and cables enablethe mechanism 140 to make necessary electrical connections with the tapeunit 100. The mechanism 140 selectively positions a secondary read-onlyhead assembly 144 adjacent to the tape 10A along the tape path at alocation between the guide rollers 114 and 116. The secondary mechanism140 also includes a secondary lead screw 146 rotatably mounted to theframe 142 and a secondary head block 148 having a follower-nut portionengaging the secondary lead screw 146 such that as the screw 146rotates, the head block 148 is translated elevationally relative to theframe 142. The secondary head block 148 rotatably mounts a lateral headshaft 150 which has one end thereof secured to the secondary headassembly 144. A guide post 152 extending from the frame 142 is followedby a guide post follower portion 154 of the secondary block 148 toprevent the block from rotating relative to the frame 142 as thesecondary lead screw 146 is rotated.

The lateral head shaft 150 is rotated by e.g. a rotary voice coil motor156 comprising a voice coil 158 attached to the shaft 150 and a statormagnet assembly 160 attached to the secondary block 148. Driving currentapplied to the voice coil 158 causes the shaft 150 to rotate betweene.g. four positions: retract, azimuth forward, longitudinal, and azimuthreverse. An optical encoder 162 provides an optical feedback signalmarking the angular location of each shaft position. The encoder 162comprises a rotating reticle plate 164 mounted to the head shaft 150 anda photo detector unit 166 mounted to the stator magnet assembly 160. Asshown in FIG. 10 a retract mechanism includes a pin 170 extendingradially from the shaft and a pin guide 172 mounted to the secondaryblock 148 adjacent the secondary head 144. The generally annular pinguide 172 includes an angled and stepped-in region 174. The stepped-inregion 174 is located such that when the shaft 150 is at the retractangle, the angled portion of the stepped-in region 174 forces the shaft150 to move axially away from the tape path and thereby retracts thesecondary head 144 from contact proximity with the tape. The stepped-inregion 174 may optionally include a detent feature for positivelymaintaining the shaft 150 at the retract position in the absence of anyrelease rotational force applied by the voice coil motor 156. A biasspring (not shown) preferably applies an axial bias force to the shaft150 to urge it axially toward the tape confronting position and awayfrom the stepped-in retract position.

FIGS. 9A, 9B, 9C and 9D show the four nominal angular positions capableof being assumed by the secondary head 144. A normal or perpendicular totape travel direction position L is shown in FIG. 9A for use in readinglongitudinally written data tracks as per the FIG. 1 format, forexample. An azimuth forward angle position is shown in FIG. 9B, and anazimuth reverse angle position is shown in FIG. 9C. These positions areused for reading azimuth track patterns shown by way of example in FIG.2. The forward angle position is assumed in reading azimuth recordtracks while the tape moves in a forward direction from supply reel 104to take-up reel 108, while the reverse angle position is assumed inreading azimuth record tracks while the tape moves in a reversedirection from take-up reel 108 back onto supply reel 104. A stepped-inretract position R is shown in FIG. 9D and represents the angularposition of the head 144 while retracted from operative proximity to thetape 10A, as shown in the FIG. 4 plan view, for example.

The secondary read-only head assembly 144 most preferably comprises fourread elements 310, 312, 314 and 316 (shown in FIGS. 13 and 14). Apreamplifier IC including a preamplifier for each of the read elementsis included on a flex circuit forming a part of the modular secondaryhead positioning mechanism 140. Suitable electrical connectors (notshown) are provided to connect the circuitry of the secondary headpositioning mechanism to circuit board electronics of the tape unit 100.

FIG. 11 illustrates one preferred form of mechanical coupling betweenthe primary lead screw 130 and the secondary lead screw 146. In thisexample, a spur gearing arrangement includes a driver gear 180 attachedto the primary lead screw 130. An idler gear 182 engages the driver gear180 and transfers rotational force to a follower gear 184 secured to thesecondary lead screw 146. The idler gear 182 rotates about a shaft 186mounted to the base 122 of tape unit 100 at a location e.g. equidistantfrom axes of rotation of the primary lead screw 130 and the secondarylead screw 146 thereby transferring rotational force imparted by steppermotor 132 to both lead screws 130 and 146.

FIG. 12 sets forth a simplified block diagram of the electronics of thetape unit 100. In pertinent part, the unit 100 includes a user datahandling section and a mechanisms section. In FIG. 12, the user datahandling functional blocks are drawn the left side of a diagrammatictape path, while the servo mechanisms functional blocks are drawn on theright side of the tape path. The user data handling blocks include auser interface 202 which interfaces the unit 100 to a host computingenvironment via a standard bus signaling convention, such as a lowvoltage differential SCSI bus 204. The interface block 202 connects toan internal user and control data bus 206. Also attached to the internalbus are a programmed data controller 208 and a block buffer memory 210.The data controller 208 regulates and controls block formatting andperforms error correction coding and decoding upon blocks written to andread back from the tape 10 (or 10A). Blocks are assembled anddeconstructed in the buffer memory 210 under direct control of the datacontroller 208. Four-channel data write/read ICs 212, 214 and 216support the multi-channel primary head assembly 126 and one of the ICs212 selectively supports read-only elements of the secondary read headassembly 144. A switch 220 switches read paths of the IC 212 from theprimary head 126 to the secondary head 144 whenever a lower densitystandard format tape is sensed within the unit 100.

The unit 100 also includes a programmed servo controller 230. The servocontroller 230 has a bus 232 enabling the data controller 208 to passcommands to the servo controller 230, and enabling the servo controllerto pass status information back to the data controller 208. In someembodiments where servo information embedded in magnetic data tracks ispresent, a connection may also exist between the servo controller 230and the channels 212, 214 and 216 via the data/control bus 206.

The servo controller 230 supervises a coarse position loop 234 whichcontrols coarse head position established by the stepper motor 134 ofthe primary head positioner mechanism 124 in accordance with trackselection values received from the data controller 208. As alreadymentioned, the stepper motor 132 simultaneously actuates lead screws 130and 146 of the primary and secondary head positioner mechanisms. Sincethe data controller 208 will learn that a particular cartridge 102 hastape recorded in accordance with a standard track format via sensor 109,coarse position will be established either with respect to primary head126 for a high density track pattern (FIG. 3) or with respect tosecondary head 146 with respect to a particular low density pattern(FIGS. 1 or 2).

The servo controller 230 also supervises a tape reel motors servo loop240 which controls operation of a supply reel motor 242 and a take-upreel motor 244 in order to establish desired tape velocity and maintaindesired tape tension during tape travel operations of unit 100.

The servo controller 230 also supervises a servo fine position loop 250which includes an amplifier 252 for controlling the voice coil motor 136of the primary head position mechanism in accordance with servoinformation provided by optical sensor 134 during high density trackformat operations with tape 10A. When a low density tape 10 is sensed bysensor 109, the servo fine position loop amplifier output is switchedfrom the primary voice coil motor 136 to the secondary voice coil motor156 via a switch 254, and the servo fine position loop 250 then employsposition information fed back from the optical sensor 166 of thesecondary head positioning mechanism in order to determine and controlthe angle of shaft 150.

By providing switches 220 and 254, duplication of electronics circuitsneeded to support both the primary write/read head 126 and the secondaryread-only head 144 is minimized.

Referring now to FIGS. 13, 14 and 15, the secondary tape head assembly144 is shown in greater structural detail. The head 144 includes agenerally elliptically shaped body 302 having a tape confronting frontface, a back face secured to shaft 150 and eight sides. The body 302 isformed of a suitable material such as non-magnetic ceramic, e.g. calciumtitanate. Other materials may also be used to form the body 302.

A tape-confronting face of the body 302 includes a raised longitudinalplateau or mesa 304 and two recessed major side surfaces or wings, aleft wing 306 and a right wing 308. The wings 306 and 308, incombination with the mesa 304 enable the body to approach the tape at avery slight tape wrap angle (e.g. one degree or less) and to be rotatedbetween positions to read back longitudinally recorded information (e.g.the FIG. 1 format) and azimuthally recorded information (e.g. the FIG. 2format) without distorting or warping the tape. Four discrete magneticread-only elements 310, 312, 314 and 316 are present at elongated wearregulated regions 318 of the longitudinal mesa 304. These elements maybe formed in accordance with techniques described in commonly assignedU.S. Pat. No. 5,426,551 entitled: “Magnetic Contact Head Having aComposite Wear Surface, and commonly assigned U.S. Pat. No. 5,475,553entitled: “Tape Head with Self-Regulating Wear Regions”, the disclosuresof these patents being incorporated herein by reference.

As shown in FIG. 14, the read-only elements 310, 312, 314 and 316 arespaced apart such that desired alignment is achieved with tracksfollowing the longitudinal format (FIG. 1) as well as tracks followingthe azimuth format (FIG. 2). A plus or minus 9.1 degree rotation isemployed for azimuth read out in accordance with the FIG. 2 standardtrack pattern. Most preferably, a distance of 0.056 inch separates theread-only elements 310 and 312 and the read-only elements 314 and 316. Adistance of 0.210 inch separates element 310 from element 314, andseparates element 312 from element 316. The mesa 304 is approximately0.02 inch across and follows a radius of curvature of approximately 0.25inch. The preferred longitudinal dimension (hl) of the head body 302along the mesa 304 is 0.407 inch which is less than the nominal width(tw, e.g. 0.5 inch) of the tape 10. The body 302 has a transversedimension (tw) of 0.300 inch as measured generally along the directionof tape travel. The wings 306 and 308 are recessed below the mesa 304 bya recess dimension (mh) most preferably lying in a range between fiveand 20 microinches. A chamfer 320 is formed along the tape facing edgesof the body 302 and follows a radius of curvature of approximately 0.5inch.

When the secondary head 144 is rotated to a tape confrontation position(FIGS. 9A, 9B or 9C) only a minimal tape wrap angle is required foroperation, most preferably about one degree of tape wrap, or less. Thisever-so-slight wrap angle suggests that the read-only head 144 floats inclose proximity to the tape with minimized contact, ensuring minimalwear and reliable long useful life as well as effective operation withboth longitudinal and azimuthal recording patterns. The minimized wrapangle also enables the head 144 to have a dimension less than the tapewidth without need for outriggers or other structure extending the majordimension of the head to be in excess of the tape width (tw) and aidsrealization of a truly compact secondary module 140.

Not all units 100 need be equipped with the secondary module 140. Inmulti-drive libraries, perhaps only one drive unit 100 need be equippedfor backward compatibility by including the module 100. However, units100 will be provided with structural and electrical features andfunctions enabling field installation of the secondary module 140,should a user require addition of the backward compatibility functionafter the unit 100 has been installed in the user environment. Also,since the secondary module 140 is truly modular, it may be removed andreplaced in the field as a unit by a service technician with minimumdisruption to operation of the drive unit 100.

It is to be understood that the particular implementations described areintended as illustrations of, and not as limiting the scope of, theclaims. It will of course be appreciated that in the development of anysuch actual implementation, numerous implementation-specific decisionsmust be made to achieve the developers' specific goals, such ascompliance with system-related and business-related constraints and thatthese goals will vary from one implementation to another. Moreover, itwill be appreciated that such a development effort might be complex andtime-consuming, but would nevertheless be a routine undertaking ofengineering for those of ordinary skill having the benefit of thisdisclosure.

Although the present invention has been described in terms of thepresently preferred embodiment, i.e., a backward compatible head andhead positioning assembly for a one-half inch tape linear digital tapedrive system, it should be clear to those skilled in the art that thepresent invention may also be utilized in conjunction with, for example,other tape drives employing different standard tape sizes and formats.Thus, it should be understood that the instant disclosure is not to beinterpreted as limiting. Various alterations and modifications will nodoubt become apparent to those skilled in the art after having read theabove disclosure. Accordingly, it is intended that the appended claimsshould be interpreted as covering all alterations and modifications asfall within the true spirit and scope of the invention.

What is claimed is:
 1. A tape unit for reading data recorded on (i) afirst cartridge having a storage tape recorded in accordance with afirst track layout and (ii) a second cartridge having a storage taperecorded in accordance with a second track layout, the tape unitcomprising: a first head assembly that reads data in accordance with thefirst track layout; a second head assembly that reads data in accordancewith the second track layout; a cartridge receiver for alternatelyreceiving the first cartridge and the second cartridge; and an indicatorfor determining which of the cartridges is inserted into the cartridgereceiver, the indicator including an optical sensor for sensing a tapehole in the storage tape of one of the cartridges.
 2. The tape unit ofclaim 1 wherein the first head assembly includes at least one, firstread element and the second head assembly includes at least one, secondread element; wherein the tape unit further comprises a head channelswitching means for switching between the at least one first readelement and the at least one second read element.
 3. The tape unit ofclaim 1 further comprising (i) a base, (ii) a plurality of tape guidesthat guide the storage tape along a tape path, and (iii) a firstpositioning mechanism secured to the base at a first location along thetape path, the first positioning mechanism positioning the first headassembly near the tape path.
 4. The tape unit of claim 3 furthercomprising a second positioning mechanism secured to the base at asecond location along the tape path, the second positioning mechanismpositioning the second head assembly near the tape path.
 5. The tapeunit of claim 4 wherein the second positioning mechanism moves thesecond head assembly between a first position in which the second headassembly contacts the storage tape in the tape path and a secondposition in which the secondary head assembly does not contact thestorage tape in the tape path.
 6. The tape unit of claim 5 wherein thesecond positioning mechanism moves the second head assembly between alongitudinal playback orientation and an azimuth playback orientation.7. A tape unit for reading data recorded on (i) a first cartridge havinga storage tape recorded in accordance with a first track layout and (ii)a second cartridge having a storage tape recorded in accordance with asecond track layout, the tape unit comprising: a first head assemblythat reads data in accordance with the first track layout; a second headassembly that reads data in accordance with the second track layout; acartridge receiver for alternately receiving the first cartridge and thesecond cartridge; and an indicator for determining which of thecartridges is inserted into the cartridge receiver, the indicatorincluding an optical sensor for sensing the presence of an optical servotrack pattern on the storage tape of one of the cartridges.
 8. The tapeunit of claim 7 therein the first head assembly includes at least one,first read element and the second head assembly includes at least one,second read element; wherein the tape unit further comprises a headchannel switching means for switching between the at least one firstread element and the at least one second read element.
 9. The tape unitof claim 7 further comprising (i) a base, (ii) a plurality of tapeguides that guide the storage tape along a tape path, and (iii) a firstpositioning mechanism secured to the base at a first location along thetape path, the first positioning mechanism positioning the first headassembly near the tape path.
 10. The tape unit of claim 9 furthercomprising a second positioning mechanism secured to the base at asecond location along the tape path, the second positioning mechanismpositioning the second head assembly near the tape path.
 11. The tapeunit of claim 10 wherein the second positioning mechanism moves thesecond head assembly between a first position in which the second headassembly contacts the storage tape in the tape path and a secondposition in which the secondary head assembly does not contact thestorage tape in the tape path.
 12. The tape unit of claim 11 wherein thesecond positioning mechanism moves the second head assembly between alongitudinal playback orientation and an azimuth playback orientation.13. A tape unit for reading data recorded on (i) a first cartridgehaving a storage tape recorded in accordance with a first track layoutand (ii) a second cartridge having a storage tape recorded in accordancewith a second track layout, the tape unit comprising: a cartridgereceiver for alternately receiving the first cartridge and the secondcartridge; an indicator for determining which of the cartridges isinserted into the cartridge receiver, the indicator including an opticalsensor for sensing a tape hole in the storage tape of one of thecartridges; a head assembly that reads data in accordance with thesecond track layout; and a positioning mechanism that moves the headassembly between a first position in which the head assembly contactsthe storage tape and a second position in which the head assembly doesnot contact the storage tape.
 14. The tape unit of claim 13 wherein thepositioning mechanism moves the head assembly to the second positionwhen the indicator determines that the first cartridge is inserted intothe cartridge receiver.
 15. The tape unit of claim 13 wherein thepositioning mechanism moves the head assembly to the first position whenthe indicator determines that the second cartridge is inserted into thecartridge receiver.
 16. A tape unit for reading data recorded on (i) afirst cartridge having a storage tape recorded in accordance with afirst track layout and (ii) a second cartridge having a storage taperecorded in accordance with a second track layout, the tape unitcomprising: a cartridge receiver for alternately receiving the firstcartridge and the second cartridge; an indicator for determining whichof the cartridges is inserted into the cartridge receiver, the indicatorincluding an optical sensor for sensing the presence of an optical servotrack pattern on the backside of the storage tape of one of thecartridges; a head assembly that reads data in accordance with thesecond track layout; and a positioning mechanism that moves the headassembly between a first position in which the head assembly contactsthe storage tape and a second position in which the head assembly doesnot contact the storage tape.
 17. The tape unit of claim 16 wherein thepositioning mechanism moves the head assembly to the second positionwhen the indicator determines that the first cartridge is inserted intothe cartridge receiver.
 18. The tape unit of claim 16 wherein thepositioning mechanism moves the head assembly to the first position whenthe indicator determines that the second cartridge is inserted into thecartridge receiver.
 19. A tape unit for reading data recorded on (i) afirst cartridge having a storage tape recorded in accordance with afirst track layout and (ii) a second cartridge having a storage taperecorded in accordance with a second track layout, the tape unitcomprising: a first head assembly that reads data in accordance with thefirst track layout; a second head assembly that reads data in accordancewith the second track layout; a base; a plurality of tape guides thatguide the storage tape along a tape path; an electromechanical, firstpositioning mechanism that positions the first head assembly near thetape path; and an electromechanical, second positioning mechanism thatpositions the second head assembly near the tape path, the secondpositioning mechanism moving the second head assembly relative to thefirst head assembly.
 20. A tape unit for reading data recorded on (i) afirst cartridge having a storage tape recorded in accordance with afirst track layout and (ii) a second cartridge having a storage taperecorded in accordance with a second track layout, the tape unitcomprising: a first head assembly that reads data in accordance with thefirst track layout; a second head assembly that reads data in accordancewith the second track layout; a base; a plurality of tape guides thatguide the storage tape along a tape path; a first positioning mechanismthat positions the first head assembly near the tape path; and a secondpositioning mechanism that positions the second head assembly near thetape path, the second positioning mechanism moving the second headassembly relative to the first head assembly, the second positioningmechanism moving the second head assembly between a first position inwhich the second head assembly contacts the storage tape in the tapepath and a second position in which the second head assembly does notcontact the storage tape in the tape path.
 21. The tape unit of claim 20wherein the second head assembly is adapted to only read the secondtrack layout.
 22. The tape unit of claim 20 further comprising anindicator for determining which of the cartridges is inserted into thecartridge receiver and wherein the second positioning mechanism movesthe second head assembly to the second position when the indicatordetermines that the first cartridge is inserted into the cartridgereceiver.
 23. The tape unit of claim 22 wherein the second positioningmechanism moves the second head assembly to the first position when theindicator determines that the second cartridge is inserted into thecartridge receiver.
 24. The tape unit of claim 22 wherein the indicatorsenses a notch in one of the cartridges.
 25. The tape unit of claim 22wherein the indicator senses a structural feature in one of thecartridges.
 26. The tape unit of claim 22 wherein the indicator senses atape hole in the storage tape of one of the cartridges.
 27. The tapeunit of claim 22 wherein the indicator senses the presence of an opticalservo track pattern on the storage tape of one of the cartridges. 28.The tape unit of claim 22 wherein the indicator senses the track layouton the storage tape to determine which of the cartridges is insertedinto the tape unit.